In recent years, scientists have developed electric vehicle with an electric propulsion system to substitute the traditional internal combustion vehicle with an internal combustion engine propulsion system, so as to resolve the problems of the price inflation of petroleum, the vast amount of exhaust emission, and green house effect. Nonetheless, electric vehicle requires a bulky on-board battery to be mounted on the vehicle. This would result in the inconvenience in charging, long charging time, and poor endurance of the vehicle. Thus, a hybrid electric vehicle that combines the advantages of high energy-saving effect of the electric vehicle and the advantages of high endurance and the convenience in fuel replenishment for the fuel vehicle, has been developed to meet the ongoing demands of ecological protection or environmental protection.
The hybrid electric vehicle is equipped with an internal combustion engine and an electric motor and is conditionally propelled by the internal combustion engine or the electric motor. The battery mounted in the hybrid electric vehicle can be charged by commercially available electric power or charged by the electricity rendered by an integrated starter generator which is driven by the operation of the internal combustion engine. When the capacity of the battery is full, the electric motor is powered by the battery. When the capacity of the battery is low, the electric motor is powered by the integrated starter generator which is ignited by the operation of the internal combustion engine, while the battery is charged by the integrated starter generator as well. As the hybrid electric vehicle uses electricity to propel the vehicle, the hybrid electric vehicle is advantageous in terms of low fuel consumption, low pollution, low energy consumption, low noise, high endurance, and high efficiency.
However, the cells in the battery, for example, the lithium iron phosphate battery (LiFePO4), is subjected to the change of temperature. If the battery is under a low temperature, for example, a temperature below zero-degree Celsius, the chargeable power of the lithium iron phosphate battery cell will be descended to 5% of its full power. This would make the hybrid electric vehicle difficult to operate under the generation mode by the integrated starter generator (ISG) of the vehicle and propel the vehicle at the same time. Also, with the change of the traction motor of the vehicle, the integrated starter generator might affect the battery. Also, in order to protect the battery, the operation of the vehicle might be unstable if the battery is detached from the inverter. Therefore, it is needed to provide an energy source to warm up the battery before the vehicle is started.